31 Euphrosyne

Euphrosyne (minor planet designation: 31 Euphrosyne) is a very young asteroid. It is one of the largest asteroids (approximately tied for 7th place, to within measurement uncertainties). It was discovered by James Ferguson on September 1, 1854, the first asteroid found from North America. It is named after Euphrosyne, one of the Charites in Greek mythology. In 2019 a small companion was discovered. It is the third-roundest known asteroid (after 1 Ceres and 10 Hygiea); this is thought to be due to having re-accreted after being disrupted by a collision, and it is not close to hydrostatic equilibrium.

Observations
Euphrosyne is a fairly dark body near the belt's outer edge. Consequently, it is never visible with binoculars, having a maximum apparent magnitude at the best possible opposition of around +10.2 (as in November 2011), which is fainter than any of the thirty asteroids previously discovered.

Euphrosyne has a high orbital inclination and eccentricity having nodes near perihelion and aphelion, Euphrosyne's perihelion lies at the northernmost point of its orbit. During perihelic oppositions, Euphrosyne is very high in the sky from northern latitudes and invisible from southern countries such as New Zealand and Chile.

Surface
Euphrosyne is a C-type asteroid with a primitive surface possibly covered by thick ejecta blanket from the collision that created its moon and collisional family. There are no deep basins. Any craters larger than 40 km in diameter must have flat floors to not be visible in the VLT images, consistent with an icy C-type composition. The lack of craters could also be due to the young age of the surface.

Mass and density
The discovery of its satellite enabled the first accurate measure of Euphrosyne's mass in 2020, at $268 km$, and thus a density of $267.1 km$. The low density suggests that Euphrosyne is half water ice if internal porosity is 20%.

Family
Euphrosyne is the namesake of a complex family of about two thousand asteroids that share similar spectral properties and orbital elements. They are thought to have arisen from a recent collision approximately 280 million years ago. All members have relatively high orbital inclinations. The second largest body in this group, 895 Helio, is most likely an interloper.

Satellite
In 2019 a small satellite was discovered, likely resulting from the same collisional event that created the family. Preliminary orbit computations indicated an orbital period of approximately 1.2 days and a semi-major axis of 670 km. VLT images indicate that the moon is 4 km in diameter, assuming it has the same albedo as Euphrosyne.